Gas hydrates are solid crystalline compounds in which gas molecules are encaged inside the lattices of ice crystals. Under suitable conditions of low temperature, high pressure and favorable geochemical regimes, gas, usually methane (CH4), will react with water to form gas hydrates. Gas hydrate is abundant along deepwater continental margins and arctic regions, trapped in hydrate accumulations or reservoirs. Current estimates of the worldwide total quantity of recoverable gas in hydrate reservoirs range between 3.1×103 to 7.6×106 trillion cubic meters in oceanic sediments. Estimates range from 2 to 10 times the amount of gas in all known remaining recoverable gas occurrences worldwide is bound in gas hydrates. While the magnitude of this resource makes gas hydrate reservoirs a future energy resource, producing from gas hydrate reservoirs provides unique technical challenges.
Natural gas hydrate reservoirs are divided into three main classes according to their geologic and reservoir conditions which can, in turn, dictate production strategies. Class 1 hydrate reservoirs comprise two zones: a hydrate-bearing interval, and an underlying two phase mobile fluid zone with free gas. Class 2 hydrate reservoirs comprise two zones: a hydrate-bearing interval overlying a mobile fluid zone with no free gas, e.g., an aquifer. Class 3 hydrate reservoirs have a single hydrate-bearing interval, and are characterized by having substantially no underlying mobile fluid zone (here after referred to as “Class 3” hydrate reservoirs). Gas can be produced from gas hydrate reservoirs by inducing dissociation using one or more of the following three main methods: (1) depressurization, (2) thermal stimulation, and (3) chemical stimulation. Depressurization methods can utilize existing production technologies and facilities but require a permeable or mobile fluid zone to produce the gas released from the dissociating hydrate. Thermal stimulation typically involves injection of hot water or steam into, the formation which requires a heat source, additional equipment and costs. Chemical stimulation can involve the injection of hydration inhibitors such as salts and alcohols which can lead to rapid dissociation and fracturing, potentially causing a breach of the reservoir. In addition, injection of hydration inhibitors requires expensive chemicals whose effectiveness is progressively reduced as released water dilutes its effect.
In terms of gas production, Class 3 hydrate reservoirs pose the largest technical challenge due to the lack of mobile fluid zones in direct contact with the hydrate interval. Gas can be readily produced from Class 1 and most Class 2 hydrate reservoirs by means of depressurization methods using conventional technology with or without a combination of thermal stimulation or chemical stimulation methods. Because of adverse permeability conditions, thermal and chemical stimulation methods have been the only production options for class 3 hydrate reservoirs, both of which are inefficient and expensive in comparison to depressurization methods.
In view of the foregoing, the contribution of the present invention resides in the discovery of a new depressurization-induced dissociation method for producing gas from Class 3 hydrate reservoirs through a well using conventional oilfield technologies, without the use of thermal or chemical stimulation.